Answer

If the frequency declines in the same way in two different places, then the same kind of natural selection must be operating.

Work Step by Step

Melanism frequency is related to natural selection for or against it, and similar changes imply similar selection pressures. (On a side note, the melanistic moth story is one of the worst for demonstrating evolution--this is well documented in the literature, including the famous journal Nature. Why textbooks keep using this example is a real puzzle.)